1. Field of the Invention
The present invention relates to computer system input devices such as a digitizers referred to as touchpads and more particularly to the systems and methods necessary to acquire signals from such input devices and to convert the acquired signals to digital codes that are transmitted to the computer system.
2. Description of Related Art
Touchpads are small digitizer based devices that are pen input devices to allow a person to write or draw upon the surface of the touchpad and have the signals and codes from a controller to be interpreted by a computer system. The touchpad digitizers may be of three types, capacitive, resistive and electromagnetic.
Referring to FIG. 1, the surface 12 of the touchpad becomes a "writing surface" for capturing the position of an pointed object 10 such as a finger, pen or stylus upon the touchpad. The touchpad signals are analog signals that will be captured by a touchpad interface circuit 28 and translated to digital codes that will be transferred to a computer system 32 on an interface 30. The interface 30 may be an industry standard serial interface, an industry standard parallel interface, or a custom interface requiring special adapter circuitry within the computer system 32 to accept the digital codes from the touchpad interface 28.
An example of a resistive touchpad is shown in FIG. 1. The resistive touchpad is made up of multiple layers of resistive films and protective layers. The protective hard coating 12 is the surface onto which the pointed object 10 is pressed upon during the writing and drawing. A first layer of resistive film 14 is attached to the protective hard coating 12 on the surface opposite the writing surface. This first layer of resistive film forms the Y-plane of the touchpad. Attached to the surface of the Y-plane resistive film 14 opposite the surface attached to the hard protective coating 12 is a second resistive film 16. This second resistive film 16 forms the X-plane of the touchpad. Finally attached to the side of the X-plane resistive film 16 is a supporting back layer 18. This back layer provides protection and mechanical support for the for the X-plane and Y-plane resistive films 14 and 16.
The touchpad interface 28 is connected through the touchpad interface lines 20, 22, 24, and 26. Each line will provide a stimulus such as a current or voltage to the periphery of the X-plane resistive film 16 and the Y-plane resistive film 14. As shown in FIG. 2, as the pointed object 10 is pressed 40 on the touchpad surface 12, the Y-plane resistive film 12 will deform and touch the X-plane resistive film 14. The X-plane resistive film can not deform because it is supported by the supporting back layer 18. This causes the Y-plane resistive film 14 and the X-plane resistive film 16 to come into contact with each other. This will cause a response in the form a change in voltage or current depending upon whether the stimulus from the touchpad interface 28 of FIG. 1 is a constant voltage or a constant current. If the stimulus from the touchpad interface 28 of FIG. 1 is a constant voltage the currents through the touchpad interface lines 20, 22, 24, and 26 will be modified according the position of the pointed object 10 on the touchpad surface 12. However, if the stimulus from the touchpad interface 28 of FIG. 1 is a constant current the voltages between the touchpad interface lines 20, 22, 24, and 26 will be modified according to the position of the pointed object 10 on the touchpad surface 12.
Referring back to FIG. 1, the touchpad interface 28 will have multiple analog to digital converters that will sense the change in the analog responses from the touchpad interface lines 20, 22, 24, and 26 and convert them to digital codes indicating the absolute position of the pointed object 10 upon the touchpad surface 12. The digital codes may be transmitted directly to the computer system across the interface 30 and translated to absolute coordinates within the computer system or the touchpad interface 28 may determine the absolute coordinates and transmit them directly to the computer system 32. For the computer system 32 to use the absolute coordinates generated by the touchpad interface 28 to control the movement of the cursor 36 upon the display screen 34.
Touchpads such as described in FIG. 1 have applications to mobile, portable, or lap top computing systems which are self contained and are powered by a battery power source. The amount of energy remaining in the a battery and the amount of energy consumed by the components of the computer system such as the touchpad and the touchpad interface are factors that must be continuously monitored and regulated to maximize the operation time of these mobile computer systems.
A class of mobile or portable computer systems are known as personal digital assistants. The personal digital assistant uses a touchpad as the primary human input interface. Handwriting must be interpreted to text and drawings to create commands and data to operate the personal digital assistant.
To interpret the hand writing accurately information regarding the pressure of the pen or stylus upon the touchpad and whether the pen is in contact with the touchpad to determine an end of a stroke for the formation of a character. Handwriting interpreting algorithms as currently applied, have only a series of recent history of the absolute coordinates of the location of the pointed object 10 of FIG. 1 upon the touchpad. From these coordinates the handwriting must be interpreted to commands and characters. If the pressure and stroke information is available handwriting interpretation could be more accurate.
U.S. Pat. No. 5,266,750 (Yatsuzuka) discloses a tablet input device and circuitry for providing stimulating voltages to the tablet input device and for sensing the response voltages from the tablet input device when the tablet input device is being pressed. The circuitry provides an OFF state wherein power to the tablet is minimized during a waiting period.
U.S. Pat. No. 5,568,409 (Neoh) assigned to the same assignee as the present invention, discloses a circuit for the implementation of the detection of a pointed object upon a touchpad and technique for the removal of power form the circuitry when the pointed object has not been detected upon the touchpad.
U.S. Pat. No. 5,287,121 (Louis, et al.) discloses a graphics input device where a stylus is used on a mechanism resembling a joy stick to provide horizontal graphics signals to a graphics processor and a feature within the mechanism to sense pressure upon the stylus to generate vertical graphics information.
U.S. Pat. No. 5,508,719 (Gervais) discusses a pressure activated pointing device for mobile or portable computing systems, where the output signals are proportional to the pressure on the pointing device.
The use of multiplexes within an analog-to-digital converter circuit is well known in the art. U.S. Pat. No. 5,446,371 (Eccleston, et al.), U.S. Pat. No. 5,150,120 (Yunus), U.S. Pat. No. 5,187,481 (Hiller), U.S. Pat. No. 4,656,585 (Stephenson), U.S. Pat. No. 4,616,325 (Heckenbach, et al.), and U.S. Pat. No. 4,196,358 (Conover, et al.) describe various applications of analog and digital multiplexing circuits for use within analog-to-digital converters.